DARPA held an industry day where it outlined some of its plans for
hypersonic aircraft of the future. The program being shown off at the industry
day was the DARPA Vulcan project. The project centers around developing an
aircraft that uses a constant volume combustion (CVC) engine capable of flight
at speeds from a standstill to Mach 4 and over.

Aviation
Week’s Ares blog reports that the first part of the program was an
introduction to the problem the program faces -- how exactly to accelerate an
aircraft from a stop to speeds fast enough to activate a supersonic-combustion
ramjet.

The program has some interesting
slides and information (PDF) from the famed Lockheed Skunk Works
HTV-3X flight demonstration vehicle that was conceived as part of the DARPA
Falcon program. One of the slides gives an idea of the size of the HTV-3X
vehicle by comparing it to the Have Blue aircraft that ended up being about 60%
of the F-117 stealth fighter.

The Lockheed HTV-3X vehicle itself has been superseded by the DARPA
Blackswift hypersonic programDailyTech has covered before. The
engine that DARPA envisions for the Vulcan project is a CVC and turbojet
combination.

According to Ares two methods can be used to achieve this type of
engine. In one method a common air inlet would be used for both the turbojet
engine that is to carry the aircraft from a stop to Mach 4 and higher speeds
and the CVC that would take over at propel the aircraft to Mach 6 and over.
This method is called turbine-based combined cycle.

The second method to achieve the engine needed is called an annular approach
and would embed a turbojet inside a CVC ramjet engine. The big challenge here
is that the turbojet would have to be cocooned when the CVC is active to
protect it from the high heat produced inside the Vulcan engine over Mach 2.

Because a turbojet capable of propelling a aircraft over Mach 4 would be
large and expensive to develop, DARPA instead wants to take a conventional Mach
2 turbojet and combine it with a CVC to get an engine capable of high Mach
speeds, but at much cheaper development costs.